In recent years, an optical recording medium such as an optical disk or the like has been rapidly prevailing as an external memory for a personal computer, a word processor, a large scale computer system or the like. As compared with a magnetic recording medium such as a magnetic disk, which has been widely used up to now, the above-mentioned optical recording medium has such various advantages that it is possessed of a larger memory capacity, a higher durability against disturbances due to vibration, electromagnetic field or the like, and a longer lifetime.
Thus, as the optical recording medium described above, there may be generally cited an optical recording medium for playback use only, an optical recording medium of write-once type and a rewritable optical recording medium. Among those, the optical recording medium of write-once type, for example a compact disk of write-once type (CD-R) or a DVD of write-once type (DVD-R), has such an advantage that it is suitable for recording or preserving particularly important information, since the user can write arbitrary information into it only once, while it is not feared that the information may be rewritten.
Hereupon, a "DVD" is referred to an optical recording medium which can record various digital signals with high density using a high efficiency encoding technique (data compressing technique) such as MPEG 2 or the like. In general, although it is much used for recording video signals, its use is not restricted to the use for recording the video signals so that it may be widely used, for example, as an external memory for a computer or the like. Hereupon, although the wording of "DVD" is not formally any abbreviation of a specified term, it may be sometimes referred to the abbreviation of the term "Digital Versatile Disk" by usage.
In general, the above-mentioned optical recording medium of write-once type, for example the CD-R, is fundamentally manufactured in accordance with the following procedure. That is, at first, a disk-shaped (doughnut-shaped) substrate is made using a transparent plastic material (for example, polycarbonate resin) or the like by means of injection molding or the like. Hereupon, during the injection molding process, a groove portion for tracking guide is formed on one of the two spreading surfaces of the substrate. Then, on the spreading surface of the substrate, on which the groove portion has been formed, there are formed a recording layer including organic pigments and a reflecting layer made of metal materials by turns. Moreover, on the reflecting layer, a protecting layer for protecting the reflecting layer is formed.
Hereupon, in the case of manufacturing the DVD-R, a covering substrate is adhered onto the reflecting layer using an adhesive.
Meanwhile, in the above-mentioned optical recording medium of write-once type, the recording layer must be formed in a very thin state (for example, the thickness of the layer must be less than or equal to 0.3 .mu.m). Thus, in the conventional process for manufacturing the optical recording medium, the recording layer is formed in such a manner that an organic pigment is applied on the spreading surface of the substrate, on which the groove portion has been formed, by means of spin coating operation or the like. However, in the above-mentioned conventional process for forming the recording layer using spin coating operation or the like, there is such a problem that it is difficult to form the recording layer of sufficiently thin state.
Thus, the inventors of the present application have developed a process for forming a sufficiently thin recording layer using the vapor deposition process which has been conventionally used in other technical fields (for example, fields for manufacturing a color filter, a semiconductor or the like). Hereupon, in the conventional vapor deposition process, for example, there is performed such a procedure including the steps of getting a substrate holder to hold the substrate using a planetary jig within a vacuum device while disposing a vaporizing source with a material for vapor deposition beneath the substrate, heating the material for vapor deposition to vaporize (sublimate) while getting the substrate holder to self-rotate and revolve, and getting vapor of the material for vapor deposition to deposit on the surface of the substrate (for example, see Japanese Patent Publication No. H8-13572).
However, if the above-mentioned conventional vapor deposition process is used for forming the recording layer of the optical recording medium of write-once type, when the organic pigment of powder state is heated, there may occur such a phenomenon that organic pigment particles having relatively small size fly apart due to their thermal expansion or their local rapid vaporization etc., the phenomenon being called splash. Thus, if the organic pigment particles, which have flied apart due to splash, adhere to the spreading surface on which the recording layer is to be formed, the recording layer is formed in the state that it wraps up the organic pigment particles. In this case, there may occur such a problem that error rate of the optical recording medium after completion of the manufacturing process is lowered (deteriorated).
Thus, in order to prevent splash described above, there has been proposed a procedure for forming the recording layer in which a pre-solidified organic pigment is used (see Japanese Laid-Open Patent Publication No. S61-105508). However, when the pre-solidified organic is used as described above, there may occur such a problem that heat conductivity is raised too excess during the heating process of the organic pigment so that the organic pigment is easily carbonized, in consequence its residue is formed so much. Hereupon, there may be thought out such a procedure as to prevent splash by using an organic pigment of pellet type. However, in this case also, there may occur such a problem that the organic pigment is easily carbonized as same as the case of using the above-mentioned pre-solidified organic pigment so that its residue is formed so much.
Further, in the conventional process for forming the recording layer by means of vapor deposition, there may be used a continuous feeding apparatus for feeding the organic pigment, which is designed to continuously feed the organic pigment of powder state contained in a hopper or the like at a constant flow rate to the organic pigment heater. In this case, if flowing property of the powder organic pigment in the hopper is lower, the organic pigment cannot be fed. Therefore, the inventors of the present application have continuously fed the powder organic pigment onto a belt conveyer while stirring the powder in the hopper using a screw so as to raise its fluidity.